[history] Add in-memory buffer of configurable number of partitions

[amitschang]

This is second of set of changes to make history perform better. For recent data (which should be most common to query) we additionaly store in-memory and query that if possible. Data are always also written out to disk and queryable when not satisfied in memory buffer.

brings UI latency from a couple seconds down to 10s of milliseconds for default query.

Resolves #183

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 96.15385% with 1 line in your changes missing coverage. Please review.	Files with missing lines	Patch %	Lines
evolver/history/standard.py	96.15%	1 Missing :warning:

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